Well, in order for the merc thing to work, you'd have to show the applicants equipment on the merc roster to verify if all the mercs have sufficient equipment for themselves. That could be very time consuming for anyone accepting mercs for a battle.
Mercs would be listed as unarmed, lightly armed, heavily armed, shiny, etc (as they would be on the map). If someone joins you "unarmed" you would know that they are unarmed and would default to using the gear you supply them with (if you choose to do so). If someone joins you "heavily armed" you would know that they are going to mean business and should be paid accordingly.
Also for my previous post, I forgot to add that you should be able to join battles at greater distances if they are your factions or allies, so that clans can still all fight together, just not across the entire map and you can only hire mercs who are relatively close. This would influence long term mercenary contracts, where mercenaries would actually follow the larger clans and actually be part of strategus.
Another cool concept would be the idea of a fully equipped order of knights riding up to two armies before a battle is launched and allowing each side to bargain for their services. Some people want to be able to play and fight for those who pay the most without making any formal alliances or signing their freedom away to contracts and treaties.
*Cough* okay one more! Imagine if ambushes could be implemented? A smaller faction of bandits could hide in a forest and ambush those who come too close. Ambushes only give the defenders ~12-15 hours of preparation instead of a whole 24 hours. (Remember AI would sign up for you automatically if you could not make the battle and hire mercs for you if you set them to do that. You still might lose, but hey, it wouldn't be a total loss.) Of course large armies (75+ or so) could not initiate ambushes regardless of how hidden they are. This would give more incentive for bandits/manhunters to play and have as much fun as larger clans.